The overall objective of the research proposed herein is to precisely estimate the mutagenic risk posed by ionizing radiation-induced structural damage to DNA. In order to accomplish this goal, we propose to use the tools of organic synthesis and genetic engineering to construct viral and plasmid DNA molecules that contain, at specific genome locations, the known products of ionizing radiation damage. These structurally altered genomes, assembled in vitro, subsequently will be introduced to the intra-cellular environment, where the radiation-induced lesions may cause mutation. Conceivably this will occur through misreplication or faulty repair of the structurally altered DNA. Finally, we will characterize quantitatively and qualitatively the mutagenic changes induced in daughter DNA molecules, and attempt to establish formal rules that relate the structure of a lesion, and possibly its persistance in DNA, with its biological effects.